1. Field of the Invention
The present invention relates to a stitch structure of double chain stitches formed in sewing products using a sewing machine, and specifically relates to a stitch structure for preventing raveling stitches caused at the end portion of stitching.
2. Description of Related Arts
Stitch structures with various modes are put into practical use so as to be suitable for types and parts of sewing products especially in the sewing using a sewing machine for industrial use. As one of these stitch structures, there is a stitch structure of double chain stitches that is represented by a D code in the Japanese Industrial Standards and further, there is also a stitch structure of flat seam stitches that is represented by the F code of the Japanese Industrial Standards and added with covered stitches with an upper cover thread to the double chain stitches.
FIG. 1 is the illustration diagram of the stitch structure of double chain stitches which indicates stitches, which appear on both faces of a cloth. As shown in the drawing, the stitches of the double chain stitches are arranged mutually in parallel on the surface 10 of the cloth 1 and composed of a plural number of needle threads 2 and 2—(3 threads in the diagram) which penetrate the cloth 1 with a pitch corresponding to a feed quantity and one looper thread 3 entwined with these needle threads 2 and 2—at the rear face 11 of the cloth 1. Further, an outline arrow mark in the diagram shows the feed direction of the cloth 1.
The needle threads 2 and 2—form respectively loops at the penetration positions at the rear face 11 of the cloth 1 and these loops form rows of thread loops 20 and 20—that are arranged to the feed direction of the cloth 1. The looper thread 3 is unreeled along the rear face 11 of the cloth 1 in a direction nearly orthogonal to the fore-mentioned feed direction and entwined with the rows of thread loops 20 and 20—to form the stitches of the double chain stitches as illustrated.
FIG. 2 is a plan view showing a state in which the needle threads are entwined with the looper thread at the rear face of the cloth. The present diagram shows the stitch structure of double chain stitches with two needles. The looper thread 3 is entwined with respective loops of the rows of thread loops 20 and 20 which the two needle threads 2 and 2 form respectively by interlooping and interlacing as illustrated in the drawing and these needle threads 2 and 2 and the looper thread 3 are collectively cut at the sewing end portion to form the stitches. However, in the stitches, since the terminal cutting portion 3a of the looper thread 3 is situated at one side of the positions at which the looper thread 3 is entwined with the final loops 2a and 2a of the rows of thread loops 20 and 20 in the mode of the interlooping, the looper thread 3 is slipped off from the final loops 2a and 2a of the rows of thread loops 20 and 20 when the terminal portion 3a of the cutting portion is pulled as shown in FIG. 2 by an arrow, and there is a problem that the slipping-off is transferred to the loop of the sewing start side (the upper side of the drawing) of the rows of thread loops 20 and 20 one after another and the stitches are raveled;
Methods of preventing raveling stitches have been conventionally proposed variously. As one of those methods, there is a method disclosed in Japanese Patent Publication No. 2879399 by the applicant of the present invention. It is a method by which when a looper for reeling out a looper thread is situated nearby a proceeding end just before the termination of sewing, the looper thread is retained at the rear side of the looper and the needle threads and the looper thread are cut after carrying out one stroke of sewing motion in this state.
FIG. 3 is a plan view of the stitch structure obtained by the method. According to the method, the looper thread 3 retained just before the termination of sewing is cut at a position at which it is pulled out from the loop 2a of one side (at the right side of the drawing and at the left side viewed from the front of a sewing machine) of the positions at which it is entwined with the final loops 2a and 2a of the rows of thread loops 20 and 20 in the mode of the interlacing, so that the stitch shown in the drawing is obtained. In this case, even if the terminal cutting portion 3a of the looper thread 3 is pulled as shown by an arrow in the drawing, the looper thread 3 is not slipped off from the final loops 2a and 2a of the rows of thread loops 20 and 20 entwined in the mode of the interlacing and the raveling stitches can be surely prevented at the stage they start.
Further, the prevention of raveling by the stitch structure shown in FIG. 3 is realized on the assumption that a state in which the looper thread 3 is entwined with the final loops 2a and 2a of the rows of thread loops 20 and 20 in the mode of the interlacing. However, since slack is liable to occur in the final loops 2a and 2a formed at the portion of termination of sewing, there is a fear that the looper thread 3 entwined with these loops 2a and 2a are slipped out to the direction of external force when the external force is applied to a direction shown by an outline arrow in FIG. 3. Since the stitches after the occurrence of slipping is the same as the stitches shown in FIG. 2, the extension of the raveling to the sewing start side cannot be stopped.
Hereat, the strength of the tension of the needle threads 2 and 2 forming the rows of thread loops 20 and 20 is selected in accordance with the coordination with the cloth 1. For example, when soft cloth or thin cloth is sewed, the tension of the needle threads 2 and 2 is often weakened for improving the quality of sewing products. Further, the kind of the needle threads 2 and 2 forming the rows of thread loops 20 and 20 is also selected in accordance with the sewing products and for example, smooth needle threads 2 and 2 are often used in sewing products thinking a great deal of the feel on the skin of a user. The fore-mentioned slack of the final loops 2a and 2a occurs easily when sewing is carried out under weak tension using the smooth needle threads 2 and 2.